Archive June 2011

When you think about hydrology, the first things that come to mind are often rain and rivers, not plants. But plants have a significant effect on how much rain actually reaches the rivers.

We see this most readily in the reduction in river flow when grassland is replaced by shrubland or forest. Low flows, mean flow and even low- to medium-size floods are diminished by forests. In an article in 2005, Kathleen Farley and colleagues pooled together results from 26 different hydrological studies. They found that annual runoff was reduced by about 44% when grasslands were planted with forests, and 31% when shrublands were planted. The effects on low flows were similar, though the proportional reductions were greater. We see the similar responses in NZ too. A 2008 MFE report includes a similar review for NZ’s studies.

These effects come about for a number of reasons, but there are several that stand out. The first two take place as the rain hits the canopy. A larger, fuller canopy tends to catch more rain and prevent it from falling to the ground. It instead evaporates back into the atmosphere. This evaporation is sped up by a taller and aerodynamically rougher forest canopy. Another reason is because trees, with more photosynthetically active foliage and deeper roots, tend to transpire more water than smaller plants, sucking more water out of the ground. Lastly, greater vegetation cover can slow runoff and speed up infiltration, which funnels more water either to the aquifer below or to evaporation and transpiration above.

What we don’t entirely know, however, are the details. How much water is intercepted by the canopy of a particular plant species? How much does that plant transpire? And how much does that plant control runoff and infiltration? While we know these numbers for some plants, there are others that we’re a lot less certain about. These details can be important if we need to assess how different land uses affect water availability and aquatic ecosystems.

Filling in these knowledge gaps is the focus of my research within the Waterscape programme. I chose to look mostly at native NZ scrub, like matagouri, manuka and kanuka. We don’t know so much about their hydrological effects, yet they can be a key feature of many of our landscapes. I’ll also look into how carbon farming or ecological succession in the conservation estate affect stream flow, and land cover effects on low flows. But more on that later.ZTMDHSWKS77D

Kaitaia’s main source of drinking water is the Awanui River. At the township, the mean annual river flow is about 6000 litres per second, most of which comes in winter and spring. February and March are typically the driest months, with a mean annual low flow (MALF) of 557 litres per second (PDF, Table 10.12). But during the 2010 drought, the river’s flow dropped precipitously low, in March reaching the lowest on record of below 320 L/s. The town had a consent to take up to 5 million litres of water per day, but normally only when the river flow was above 460 L/s. With conditions so dire, water restrictions had long been in place, and an emergency directive to continue taking water was granted. But while this drought illustrates the possible year-to-year variations about the average, how might the average flow conditions change as climate changes?

This was a question I was posed last year, as part of a project to examine the potential impacts of climate change on Maori communities. The challenge was to answer the question quickly. This essentially ruled out simulation modelling, so I turned to statistics.

I knew I had a long record of flow for the river, as well as of rainfall and temperature in the area. I also knew that projections of rainfall and temperature change had already been documented for the area on a seasonal basis [see MFE]. Lastly, as the area never experiences snowfall, it’s fair to say that the nature of the hydrological processes will not substantially change. Thus I refined the question to become: What were seasonal Awanui River flows like in the past during conditions that resemble climate change projections of the future?

Answering the new question meant that I had to construct a statistical model of mean seasonal flow as a function of mean seasonal temperature and rainfall. I kept the model as simple as possible, to avoid giving us a false sense of certainty, but not so simple as to violate established hydrological principles. I also made sure that the forecasts I was making, which were of average conditions, were within the historical range of variability, and so I was not stretching the model beyond breaking point.

In the end, the mid-range projections of the seasonal flows pointed to reductions throughout the year, but mostly in winter and spring (see the coloured bars in the figure below). The error bars for the two forecast periods indicate the range of uncertainty from the climate modelling. This uncertainty means that projections for the all-important summer flows ranged from a substantial decrease (-45% by 2090) to a moderate increase (+25% by 2090). As for winter and spring, reductions were projected across the board.

This has important implications for water resources planning for the future. With a reduction in mean flows during spring, conditions would be increasingly primed for a summer hydrological drought. And while mean summer flow could increase, the chances are higher that it will decrease, perhaps by a lot. This points to more low flows in the Awanui River, and probably more extreme low flows at that. How Kaitaia should adapt depends a lot on the capacity and aspirations of the community, but there are two good candidates. One is the ramping up and institutionalisation of water conservation within the town (e.g., dual flush toilets, less water-intensive gardens, household rainwater storage). The other is a greater reliance on reservoirs to store winter flow for the specific purpose of alleviating summer droughts. A risk with the second option, however, is that during times of plenty it could be used to grow demand, which would defeat the purpose of drought resilience.

Despite what they say on Broadway, the rain in Spain does not stay mainly in the plain. Most of New Zealand’s rain doesn’t fall in the plains either, but in the mountains, particularly the Southern Alps. This is why the West Coast is so wet. The plains are in fact quite dry in comparison. In an earlier post, I mentioned how much precipitation falls on each region, but now I want to zoom in further.

Look at the map of rainfall below (actually, it’s total precipitation). While the average annual precipitation depth over the West Coast is 5.5 m, some parts of the alps get a lot more. You can also make out a small circle of high precipitation around Mt Taranaki on the west coast of the North Island. The driest places, in terms of precipitation, are inland Otago and Canterbury’s Mackenzie Basin. The Canterbury Plains are pretty dry too, as well as the pocket of land around Napier and Hastings. So just as each region receives more or less rainfall than the national average, so too does each catchment and even each valley.

Now look at the topographic map. You’ll see a strong correspondence with the rainfall map: mountainous locations tend to receive more rain than their lower and flatter neighbours. This is known as orographic precipitation, from the Greek word for mountain, ‘oros’. When moisture-laden air approaches hills and mountains, it is forced up and over the obstructing terrain. As it rises, the air cools and expands, so much so that the air temperature may drop to the dew point. (In thermodynamics, this process is called adiabatic cooling.) When this happens, the water vapour in the air starts to condense into clouds; when the cloud droplets are large enough, it starts to rain or snow. And so, broadly speaking, where the topography rises sharply, the more it will tend to rain, provided that the air is sufficiently moist. This casts a rain shadow over the downwind areas, which is why central Otago, the Mackenzie Basin and the Canterbury plains are so dry.

Of course, this picture is just an annual average and the topography is only one of the many factors at play here. In future posts we’ll talk a bit more about how the picture changes over time: among seasons, years and even decades. We’ll also describe the monitoring network that paints this picture for us, how this picture has changed in the past, and how we are striving to improve it even further.

Hydrology is the study of the presence and movement of water over and under the Earth’s surface. Like many sciences, hydrology is often partitioned into sub-disciplines: surface hydrology and groundwater hydrology. This separation shows up in hydrology courses at universities, in research programmes and organisations, and in the way resource management agencies manage water. It’s a natural separation, but it causes problems because the two subjects get treated separately, despite them being closely linked. Hydrology is divided this way in part because of the historical evolution of the discipline, and in part because the techniques used by the two are quite different.

Hydrology doesn’t stand still. Like other sciences, it evolves as scientists ask new questions and as new technologies became available. One of the big questions in hydrology at the moment is how to link surface hydrology and groundwater hydrology.

To answer this question in the context of New Zealand we have established the MSI-funded Waterscape programme. We’ve brought researchers together from both inside NIWA and outside (Aqualinc Research, GNS) to tackle the problem. Our goal is to construct the overview picture of the water cycle that links together surface and groundwater.

We do already have the water cycle as a powerful concept for linking together the many water pathways. But when it comes to making practical decisions about water, water managers often don’t have the hard numbers on what water goes where, to let society make good decisions. Our top priorities for Waterscape are to gather new data to fill some of the more important gaps, and to develop tools to help scientists and resource managers get better understanding of the combined surface-groundwater resource.

The data gaps we’ll be filling include: the role of native scrub on catchment hydrology, the role of soils in conveying water from hills to rivers, the role of groundwater in feeding rivers, and the use of water by crops.

A big part of why Waiology was set up is to let more people know how the research progresses and what we find, so stay tuned.

The amount of water that falls on New Zealand each year is about 560,000 million m3. Lumped together as ‘precipitation’, this is mostly rain, but also snow, hail and even some graupel. That’s enough water to cover the country 2.1 metres deep, or to fill Lake Taupo over 9 times. More than most countries, but not all.

How much of this precipitation makes its way to the sea, instead of evaporating along the way, is less clear. This number is important because it tells us about how much freshwater is available to the environment and us. The most commonly cited report to date, released by Statistics NZ, puts this at about 79%, but there is good reason to believe this estimate to be on the high side. A study published in 1972 reported 72%. Unpublished results from another in 2006 reported 66%. And the Statistics NZ analysis is being updated at this very moment — we’ll let you know the results when they’re available.

And just as New Zealand has more or less water than other countries, the same goes for our regions. Check out the figures below. According to the StatsNZ report again, the West Coast is by far the wettest, on account of the Southern Alps blocking the mean westerly airflow, followed by Southland. Otago is the driest, with its heart sheltered from high rainfall. If you spread West Coast’s precipitation evenly over its surface, you’d be swimming in water 5.5 metres deep. For Otago, you’d just be standing in water 1.1 m deep. Because some of this water evaporates, the equivalent depth of freshwater spread over the regions is lower — 5 m for the West Coast and 80 cm for Otago.

Now remember that these are estimates. We can’t measure all the rainfall everywhere or gauge every stream — there’s not enough money and we don’t really need to. Instead, we design monitoring networks to capture the most important information, make informed judgments based on what data we do collect, and rely on physical principles to fill in the gaps. As more data are collected, and as the science advances, these estimates get refined. A lot of what Waiology will cover is how we refine these estimates and what we find out.

Water (‘wai’ in te reo Maori) is one of NZ’s most precious resources. We drink it, we eat food grown with it, we power our homes and businesses with it, we play in it, and we are mentally and spiritually uplifted by it and by what it brings. But despite being relatively water-rich on an international stage, what we want from this water outstrips what we actually get.

That’s where hydrologists come in. Our job is to work out what’s going on with the water and let you know. How much water is there, when and why? What might happen if we cut down those trees or pump a well? Is there enough water to support land use change, and what might happen with climate change?

Giving you the answers, as best we can, is why we’ve set up Waiology – a blog on the science of New Zealand’s water cycle. Waiology is a group effort by a team of hydrologists at NIWA. We’ll give you a mix of what we already know about NZ’s hydrology, and invite you to follow our on-going research, much of which will be centred around a MSI-funded grant we call Waterscape.

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